Crystallization apparatus



Nov. 9, 1965 J. D. RATJE ETAL CRYSTALLIZATION APPARATUS Original FiledApril 3, 1957 2 Sheets-Sheet l CONTROL APPARATUS I i i FEED 39DIELECTRIC i 32 33 PROBE I CHILLER 3 34 as {41 FIG. 2

431/ INVENTORS J.D. RATJE H.W. TOMPKINS 72 7 ve c 7 BY Nov. 9, 1965 J.D. RATJE ETAL 3,216,212

CRYSTALLIZATION APPARATUS Original Filed April 3, 1957 2 Sheets-Sheet 286.6 MOL 7 MVP 63.7 MOL.7 MVP SOLIDS CONTENT, MOL "7 N I I 5.5 6.0 6.57.0 7.5 DIELECTRIC CONSTANT MVPMEP SYSTEM FIG. 5

INVENTORS J.D. RATJE H.W. TOMPKINS United States Patent 3,216,212CRYSTALLIZATION APPARATUS John D. Ratje, Gallup, N. Mex., and Harold W.Tompkins, Phillips, Tex., assignors to Phillips Petroleum Company, acorporation of Delaware Original application Apr. 3, 1957, Ser. No.650,419, now Patent N0. 3,093,649, dated June 11, 1963. Divided and thisapplication July 27, 1962, Ser. No. 212,822

6 Claims. (Cl. 62-123) This is a divisional application of our copendingapplicatlon Serial No. 650,419, filed April 3, 1957, now Patent No.3,093,649.

This invention relates to the separation and purification of componentsof liquid multi-component mixtures. In one aspect, it relates to theseparation and purification of components of a liquid multi-componentmixture by fractional crystallization. In another aspect, it relates tomeans for controlling the solids content of a slurry supplied to thepurification column of fractional crystallization apparatus. In stillanother aspect, it relates to a method for controlling the solidscontent of a chiller used in fractional crystallization apparatus.

The separation of chemical compounds by means of crystallization findsmany applications in industrial installations. While many separationscan be made by distillation or solvent extraction, there are cases wherethese methods are impracticable or impossible, and the desiredseparation can be elfected more advantageously by means ofcrystallization. Thus, in the case of chemical isomers having similarboiling points and solubilities, or materials having relatively highboiling ranges, or thermal unstable substances, separation bycrystallization may be the only method which can be advantageouslyemployed.

As well as offering in many cases perhaps the only practicable method ofseparation, the crystallization method oifers the further advantage ofbeing the only separation method which theoretically gives a pureproduct in a single stage of operation. In actual practice, however, thecrystals obtained from a solution of several components will be impurebecause of the occlusion of mother liquor Within the crystalinterstices. In the conventional fractional crystallization processes,the crystal yield from one-batch crystallization is redissolved in asolvent or remelted and again crystallized to effect furtherpurification. The recrystallized product will have less impurities sincethe concentration of impurity in the new liquor is less than in theprevious liquor of crystallization. Such processes require a largeamount of equipment and floor space for their operation with resultinghigh operating exepnditures in terms of labor and equipment costs.Furthermore, in these types of processes, the purity of the product islimited by the number of stages through which the process is carried.

More recently, a continuous method of separating and purifying liquidmulti-component mixtures has been advanced which overcomes thedisadvantages of conventional fractional crystallization processes. Thismethod involves cooling a liquid multi-component mixture from which theseparation is to be made so as to form crystals of at least the highermelting component and thereafter supplying the resulting slurry to acrystal separation and purification column. In this column, crystals areseparated from mother liquor and then introduced into a purificationsection in one end of which a melting section is maintained. Thecrystals are moved through the purification section toward the meltingsection where the 3,216,212 Patented Nov. '9, 1965 crystals are melted,and a portion of the melt is Withdrawn as product. The remainder of themelt is displaced countercurrently to the movement of crystals and inintimate contact therewith so as to remove occluded impurities.

When practicing the above-described crystal separation and purificationmethod, it has been found to be desirable to supply to the separationand purification column a slurry which has a constant solids content.When operating in this manner, the production of a high purity productin high yields is greatly facilitated. It also makes possible a steadyoperating procedure in which fluctuations in product yield and productpurity are reduced to a minimum. Furthermore, when practicing theabovedescribed method, it is often desirable to maintain the solidscontent of the slurry as high as possible while still producing a slurrywhich is capable of flowing. In order to maintain a high solids contentslurry, close control over the refrigeration process whereby the solidsare formed is required. Several methods have been advanced forcontrolling the operation of the crystal forming means so as to providea slurry having a constant solids content. One method proposescontrolling the refrigeration requirements in accordance with thetemperature of the slurry. However, this method has not been entirelysatisfactory in systems in which there are large changes in solidscontent with small changes in temperature. In accordance with theinstant invention, a method is provided for controlling the solidscontent of a chiller slurry, which can be advantageously used with allsystems, particularly with those exhibiting large changes in solidscontent with small changes in temperatures.

It is an object of this invention to provide improved fractionalcrystallization apparatus.

Another object of the invention is to provide an improved process forthe separation of components of liquid multi-component mixtures.

Still another object of the invention is to provide fractionalcrystallization apparatus comprising an improved crystal forming meanshaving associated therewith means for controlling the solids content ofa crystal slurry.

Still another object of the invention is to provide apparatus forcontrolling the refrigeration requirements of a chiller in response to ameasurement of the dielectric properties of the solids slurry formed inthe chilller.

A further object of the invention is to provide an improved chillerwhich includes means for controlling the solids content of the slurryformed therein in response to a measurement of the dielectric propertiesof the slurry produced in the chiller.

A still further object of the invention is to provide a method forcontrolling the refrigeration requirements at a chiller so as to obtaina slurry having a desired solids content.

Other and further objects, advantages and features of the invention willbecome apparent to one skilled in the art upon consideration of theaccompanying disclosure.

Broadly speaking, the present invention is directed to the separationand purification of components of liquid multi-component mixtures andinvolves controlling the operation of the chiller or crystallizer so asto provide a slurry having a desired constant solids content. Thecontrol method of this invention is based upon the discovery that thedielectric constant of a slurry obtained by cooling any given feedmixture is a function of the amount of solids contained in the slurry.In 'one embodiment, in a process which comprises passing a slurry3-Ethylpentane- 3-Ethyl-2-methylpentane of solids in mother liquor intoa purification zone, separating mother liquor from the slurry within thepurification zone, moving solids through the purification zone toward amelting zone, melting solids in the melting zone, displacing a portionof the resulting melt into the moving solids, and recovering a purifiedproduct from the melting zone, the invention resides in the improvementcomprising measuring the dielectric properties of the slurry passed intothe purification zone, and adjusting the solids content of the slurry inresponse to this measurement.

The process described herein can be advantageously employed inconjunction with practically any system to which fractionalcrystallization is applicable in order to increase the efficiency of theseparation. Thus, the process and apparatus of this invention areapplicable to a vast number of simple binary and complex multi-componentsystems. The invention is particularly applicable to the separation ofhydrocarbons which have practically the same boiling points and are,therefore, difficult to separate by distillation. Where high boilingorganic compounds are concerned, separation by distillation is oftenundesirable because many such compounds are unstable at hightemperatures. One particular advantageous application of the processlies in its use with systems which exhibit large changes in solidscontent with small changes in temperature, e.g., witha mixturecontaining 85 mol per cent or more Z-methyl-S-vinylpyridine, with normalparafiins, or with a system containing a high percentage of water. Inorder to illustrate some of the systems to which the invention isapplicable, the following compounds are grouped with respect to theirboiling points:

Group A B.P., F.P.,

Benzene 80 5, 5 n-Hexane 69 94 n-He e 98. 52 90. 5 Carbon tetrachlonde77 22. 8 Acrylonitrile- 79 82 Ethyl alcohol 78. 5 117. 32,2-Dimethy1pentane 79 125 3,3-Dimethylpentane 86 Meth 79. 6 86. 4 Methy79. 9 87. 5 Methyl acrylate. 80. 5 1,3-Cyclohexadiene 80. 5 982,4-Dimethylpentane 80. 8 123. 4 2,2,3-Trimethylbutane. 80. 9 25Cyclohexane 81. 4 6. 5 Acetonitrile- 82 42 Cyclohexene 83 103. 7Z-Methylhexane. 90 119 B-Methylhexane- 89. 4 119. 4

Group B B.P., C F.P., 0.

Methyl cyclohexane 100.3 -126. 3 Cyclohexane 81. 4 6. 5 n-Heptane 98. 5290. 5 2,2,4-Trimethylpentane 99. 3 -107. 4 Nitromethane 100 29 p-Dinxane101. 5 ll.v 7 2-Pentanone-- 101. 7 77. 8 2-Methyl-2-buta 101. 8 -11. 92,3-Dimethylpenta 89. 4 93. 3 94. 5

Group C Toluene Methylcyclohexane 2 ,2 ,3 .3-Tetramethyl butane2,5-Dimethylhexane 2 ,4-D imethy 2,3-Dimethylhexane 3,4-DimethylhexaneGroup D 13.]? C. F.P C.

.AniIinP 184. 4 6. 2 Tnlnene 110. 8 95 -Benmne 80. 5.

Group E B.P., C. F.P., 0

Carbon tetrachloride 77 22. 8 Chloroform 61 -63. 5 CS; 46.3 l08.6Acetone 56. 5

GroupF B.P., O. F.1.,C

Orthoxylene 144 27. 1 Metaxylene 138. 8 47. 4 Para-xylene 138. 5 13. 2

Group G B.P., C. F.P., C

Ortho-cymene 175. O 73. 5 Meta'eymene 175. 7 -25 Para-cymene 176. 0 73.5

Group H B.P., C. M.P., C.

Dirnethyl phthalate 282 5. 5 Dimethyl isophthalate 124 (I? 67 mm.Dimethyl terephthalate 288 140. 6

GroupI B.P., C. M.P.,C.

Ortho-nitrotoluene 222. 3 12. (15 Meta-nitrotoluene 231 15: 5Para-nitrotoluene 238 51. 3

Mixtures consisting of any combination of two or more of the componentswithin any one of the groups can be resolved by the process of theinvention, as can mixtures made up of components selected from differentgroups. For example, benzene can .be separated from a benzene-uhexane ora benzene-n-heptane mixture in which the benzene is present in an amountgreater than the eutectic concentration. In the same manner, para-xylenemay be readily separated from a mixture of paraand metaxylenes or frompara, meta-, or ortho-xylenes. Benzene can also .be separated from amixture thereof 'with toluene and/or aniline. Multi-component mixtureswhich can be effectively resolved so as to recover one or more of thecomponents in subtsantially pure form include mixtures of at least twoof 2,2-dimethylpentane, 2,4-dimethylpentane, and mixtures of at leasttwo of carbon tetrachloride, chloroform, and acetone. The invention isalso applicable to the separation of individual components from a systemof cymenes.

This invention can also be utilized to purify naphthalene, hydroquinone(1,4-benzenediol), paracresol, paradichlorobenzene, and such materialsas high melting waxes, fatty acids, and high molecular weight normalparafiins. The invention can also be used to resolve a mixturecomprising anthracene, phenanthrene, and carbazole. Furthermore, theinvention can be used to separate durene (1,2,4,S-tetramethylbenzene)from C aromatics.

It is not intended, however, to limit the invention to organic mixtures,but rather it is applicable to inorganic mixtures as well, and offers apractical method of separating two inorganic components between whichsolvates or hydrates are formed. Examples of inorganic systems to whichthis invention is applicable are those for the recovery of pure salts,such as ammonium nitrate, and of anhydrous salts from their hydrates.

In certain cases, it is also desirable to recover the mother liquorseparated from the crystals as a product of the process. This situationarises where it is desired to increase the concentration of a dilutesolution. This aspect of the invention is especially applicable to theproduction of concentrated food products which involves primarily theremoval of water fromthese products. Ac-

cordingly, by utilizing the process of this invention, water can beremoved from fruit juices such as grape, orange, lemon, pineapple, appleand tomato. It is also possible to concentrate vegetable juices andbeverages such as milk, beer, wine, coffee and tea by this method. Thedesired degree of concentration can be closely controlled by varying theamount of liquid passed as reflux into the moving mass of crystals. Thisaspect of this invention is in general applicable in those instanceswhere it is desired to increase the concentration of a solution byremoving at least a portion of the solvent therefrom.

For a more complete understanding of the invention, reference may be hadto the following description and the drawing, in which:

FIGURE 1 is an elevational view of fractional crystallization apparatusillustrating the present invention;

FIGURE 2 is a detailed view of the dielectric measuring probe used inthe apparatus of FIGURE 1;

FIGURE 3 is a sectional View taken along line 33 of FIGURE 2;

FIGURE 4 is a schematic representation of the control apparatus ofFIGURE 1; and

FIGURE 5 is a graph illustrating the ralationship between dielectricconstant and solids content of a 2-methyl- S-Vinylpyridine feed stream.

Referring to FIGURE 1 of the drawing, an elongated crystal andpurification column is closed at its upper and lower ends by closuremembers 11 and 12, respectively. The upper end of column 10 is providedwith a compacting means, such as impervious piston 13, connected by aconnecting rod 14 to a hydraulic piston 16 in hydraulic cylinder 17.Lines 18 and 19 serve to pass hydraulic fluid alternatively into and outof cylinder 17 so as to drive piston 16 which in turn causes themovement of piston 13. It is within the scope of the invention to use aporous piston in which case the piston serves also as a filtering means.When operating the column with a porous piston, an outlet line isconnected to the upper end of column 10 for removal of liquid therefrom.Filter section 21, disposed in an intermediate portion of column 10,comprises a filter medium, such as a filter screen 22, surrounded byjacket 23. Jacket 23 has a line 24 connected thereto for withdrawal ofliquid from the filter section. The portion of column 10 below filtersection 21 and in communication therewith comprises purification section26. A heat exchange means is positioned in the lower end of column 10 inorder to provide a crystal melting section in that end of the column. Asillustrated, the heat exchange means is a coil 27 through which a heattransfer medium is circulated. However, it is not intended to limit theinvention to the specific heating means shown, for other suitable meansmay be employed. For example, an electrical heater may be positionednext to closure member 12, a coil may be disposed around column 10 atits lower end, or an electrical bayonet type heater may be provided toextend into the end of the column. A liquid outlet line 29 provided witha valve 31 is connected to the end of column 10 to provide means for theremoval of melt or a mixture of melt and crystals from the meltingsection.

Feed inlet line 32 leading from a source of feed material, not shown,and containing a pump 33 is connected to the inlet end of chiller 34.Chiller 34 may be any conventional type of refrigerating or crystalforming means such as a scraped surface chiller. As illustrated, thechiller comprises a cylindrical member 36 having positioned herein meansfor moving crystal slurry formed therein through the chiller, such as anauger connected to a motor 37. The cylindrical member is closed at itsouter end while its other end is connected to column 10 at a point abovefilter section 21. Cylindrical member 36 is encompassed by a jacket 38through which a coolant, such as a brine solution or an alcohol-watermixture, is continuously circulated by means of inlet line 39 and outletline 41 connected to the jacket. Line 39 contains a flow control means,such as motor valve 40, to provide means for controlling the rate atwhich the coolant is introduced into the jacket. It is also within thescope of the invention to position this valve in outlet line 41. Thus,when the desired refrigeration is accomplished by the evaporation of anormally gaseous material, e.g., a light hydrocarbon, such as propane,valve 40 is positioned in line 41 to provide means for controlling therate of evaporation.

In the operation of the apparatus of FIGURE 1, a liquid feed, which maybe a liquid multi-component mixture containing components of differentmelting points, is pumped from a source, not shown, by pump 33 intochiller 34. Chiller 34 is maintained at a temperature low enough tocrystallize a portion of one of the components and form a slurry ofcrystals in mother liquor. This is accomplished by circulating asuitable coolant through jacket 38 by means of lines 39 and 41. Asdiscussed hereinbefore, it has been found that for improved operation ofthe crystal purification apparatus, it is important to provide a crystalslurry having a predetermined constant solids content. In accordancewith the control method of this invention, the dielectric properties ofthe crystal slurry introduced into crystal purification column 10 arecontinuously measured by means of probe unit 42 inserted in cylindricalmember 36. It is to be understand that the auger, in cylindrical member36, does not extend into that portion of the cylindrical member in whichthe probe unit is disposed. The particular probe unit and the controlapparatus 43 used in conjunction therewith is described in detail incopending U.S. patent application Serial No. 500,466, filed April 11,1955, by W. D. Peters, and now issued as Patent No. 2,800,394. While itis not intended to limit the instant invention to any particular controlapparatus and means for measuring the dielectric properties of thecrystal slurry, the apparatus described in the cited patent applicationcan be advantageously used in the practice of the instant invention.

Probe unit 42 is shown in detail in FIGURE 2 of the drawing. Cylindricalmember 36 has an opening 44 in the side wall thereof which receives thedielectric probe unit. A threaded shoulder 45 which is attached tochiller 34 surrounds opening 44. The probe unit can be in the form of abase 46 which is threaded to shoulder 45. Base 46 supports a pair ofspaced apart plates 47 and 48, which extend into cylindrical member 36.As illustrated in FIGURE 3, these plates can be streamlined to minimizeresistance to the flow of slurry through the cylindrical member. Plates47 and 48 are in electrical contact with base 46, which can be groundedelectrically. A center plate 49 is mounted between plates 47 and 48 byelectrically insulating spacers 51. The three plates are held inassembled relation by screws 52 which extend through spacers 51. Anelectrical lead 53 is attached to plate 49 and extends outwardly throughan opening in base 46. To prevent leakage of material from cylindricalmember 36, suitable sealing means are provided about base 46. Spacedplates 47, 48 and 49 thus form an electrical condenser wherein theslurry passed through cylindrical member 36 constitutes the dielectricmaterial between the plates.

The dielectric properties of the slurry in cylindrical member 36 aremeasured by measuring the capacitance of the condenser formed by plates47, 48 and 49. As shown in FIGURES 2 and 4, this unit is designated ascapacitor 56. One terminal of capacitor 56 is connected to ground andthe second terminal thereof is connected to the first terminal of aresistor 57 as seen in FIGURE 4, which shows the details of controlapparatus 43 of FIGURE 1. The second terminal of resistor 57 isconnected to the first terminal of a source of alternating current 58,the second terminal of which is grounded. A rectifier 59 and a resistor61 are connected in series relation with one another and in parallelwith capacitor 56. A capacitor 62 is connected in parallel with resistor61. The junction be- .motor 68.

tween rectifier 59 and resistor 61 is connected to the first inputterminal of an amplifier 63. A second rectifier 64 and a potentiometer66 are connected in series relation with one another between the firstterminal of current source 58 and ground. A capacitor 67 is connected inparallel with potentiometer 66. The contactor of potentiometer 66 isconnected to the second input terminal of amplifier 63.

As thus far described, the circuit is a modified form of a Wheatstonebridge. Resistor 57 and capacitor 56 can be considered as constitutingtwo arms of the bridge circuit while the upper and lower portions ofpotentiometer 66 can be considered as the other two arms of the bridge.Current source 58 is connected across the first opposite terminals ofthe bridge. Amplifier 63, which serves as a bridge unbalance indicator,is connected between the second opposite terminals of the bridge.Rectifiers 59 and 64 are provided to convert the alternating currentbridge unbalance signal into a corresponding direct current which isapplied to the input of amplifier 63. The output terminals of amplifier63 are connected to a reversible servo motor 68, the drive shaft ofwhich is mechanically coupled to the contactor of potentiometer 66.Amplifier 63 can include a converter in the input circuit thereof toconvert the input direct current signal into a corresponding alternatingsignal. Servo motor 68 can be a reversible two phase motor which rotatesin a first direction when a signal of first phase is applied thereto androtates in a second direction when a signal 180 degrees out of phasewith the first signal is applied thereto. Thus, any electric unbalancein the bridge circuit drives motor 68 in a direction so that thecontactor of potentiometer 66 is moved until the unbalance signal isagain zero. Any change in the dielectric properties of the slurry incylindrical member 36 thus results in the movement of the contactor ofpotentiometer 66. The magnitude and direction of this movement arefunctions of the direction of and the magnitude of the change of thedielectric properties of the slurry flowing through cylindrical member36.

The drive shaft of motor 68 is also mechanically couple-d to thecontactor of potentiometer 71. A voltage source 72 is connected acrossthe end terminals of potentiometer 71. The contactor and one endterminal of potentiometer 71 are connected by respective leads 73 and 74to the input terminals of a potentiometer controller 76. Controller 76provides an output air pressure representative of the electrical signalapplied thereto. This air pressure resets a rate of flow controller 77which adjusts valve 40 in coolant inlet line 39 of FIGURE 1 in responseto the rotation of It is to be understood that the air signal fromcontroller 76 can be used, if desired, to control valve 40 directly.

As discussed hereinbefore, the control method of thls invention isdependent upon the discovery that the dielectric properties of a slurryare a function of the solids content of that slurry. In FIGURE of thedrawing, two curves are shown which illustrate the relationship betweendielectric constant and solids content of two Z-methyl-S-Vinylpyridinefeed streams. The dielectric constant measurements were obtained byusing a Sargent Chemical Oscillometer, Model V, manufactured by E. H.Sargent & Company, Chicago, Illinois. Similar measurements have beenobtained in the case of other systems, including systems containingnormal parafiins, such as dodecane or tridecane, and systems containingaromatics, such as para-Xylene. In the operation of the control system,rate of flow controller 77 is given an initial setting which providesthe coolant circulation rate necessary to obtain a slurry having adesired solids content. It is seen from FIGURE 5 that a slurry having acertain solids content has a specific dielectric constant. If thedielectric constant of the slurry changes, controller 76 operates as asto reset rate of flow controller 77 which in turn adjusts valve 40,thereby altering the rate of flow of the coolant through the chillerjacket. For example, if the measured dielectric constant decreases,indicating an increase in solids content, valve 40 is closed by apreselected amount, thereby cutting back on the coolant circulationrate. Conversely, if the measured dielectric constant increases,indicating a decrease in the solids content of the slurry, valve 40 isopened by a preselected amount so as to increase the coolant circulationrate. It is to be understood that with certain systems the dielectricconstant of the slurry increases with an increase in solids content anddecreases with a decrease in solids content.

By proceeding as described hereinbefore, it is possible to supply aslurry having a desired constant solids content to the upper end ofpurification column 10. Upon introduction into column 10, the slurry ismoved downwardly by means of piston 13 into filter section 21. Piston 13is forced downwardly and upwardly by means of hydraulic piston 16 whichis moved in response to hydraulic fluid introduced intdand withdrawnfrom hydraulic cylinder 17 through lines 18 and 19. By operating in thismanner, piston 13 on its compression stroke forces crystals downwardlythrough column 10 while on its back stroke crystal slurry is allowed topass into the column from chiller 34.

Within filter section 21 mother liquor is separated from the crystalsand removed from the column through line 24. The crystals thereaftercontinue their movement as a uniform mass downwardly through the columnas a result of the force asserted thereon by piston 13. Crystals onapproaching the end of column 10 enter the melting zone maintained inthe end of the column by heating means 27. The melting zone ismaintained at a temperature at least as high as the melting point of thecrystals by continuously circulating a heat exchange medium through thecoil of the heating means. On reaching the melting zone, at least aportion of the crystals are melted, and a portion of the resulting meltis displaced upwardly as a reflux stream into the downwardly moving massof crystals. The reflux stream on contacting the crystals upstreamcrystalwise of the melting zone displace occluded impurities from thecrystals by refreezing thereon. A liquid stream comprising displacedimpurities is removed from column 10 through filter section 21 by meansof line 24. A substantially pure product in the form of melt or amixture of melt and crystals is withdrawn from the melting zone throughline 29.

While the instant invention has been described in conjunction with aparticular crystal purification column, it is not intended to so limitthe invention. The invention is broadly applicable to any purificationcolumn which utilizes a displaced reflux stream to obtain a high purityproduct. One type of crystal purification apparatus with which theinstant invention can be advantageously used is described in copendingUS. patent application Serial No. 494,866, filed by R. W. Thomas onMarch 17, 1955, now US. Patent No. 2,854,494. Furthermore, while theapparatus of FIGURE 1 has, for the sake of clarity of understanding,been illustrated and described as occupying a substantially verticalposition, it is not intended to so limit the invention. It is to beunderstood that the apparatus can be otherwise disposed withoutdeparting from the spirit or scope of the invention. Thus, theseparation and purification column can be positined horizontally or thecolumn can be operated vertically with the melting zone in the top ofthe column rather than in the bottom as illustrated.

A more comprehensive understanding of the invention may be obtained byreference to the following illustrative example which is not intended,however, to be unduly limitative of the invention.

Example A feed material containing about 86.6 mol percent2-methyl-S-vinylpyridine (MVP), the remainder being primarilyZ-methyI-S-ethylpyridine (MEP), is charged to the chiller of fractionalcrystallization apparatus similar to that of FIGURE 1 at a temperatureof about 70 F and a rate of 19,965 pounds per day. In this example,propane is used as the cooling medium and valve 40 is positioned in thecoolant outlet line in order to control the rate of evaporation of thismaterial. It is desired to control the operation of chiller so that theslurry upon introduction into the separation and purification columncontains 40 mol percent solids.

As seen from an examination of FIGURE 5, an MVP slurry obtained from theabove-mentioned feed and having a solids content of 40 mole percent hasa dielectric constant of about 5.4. Rate of flow controller 77 is givenan initial setting such that the amount of propane evaporated gives therefrigeration required to produce a slurry containing 40 mol percentsolids. The index of potentiometer controller 76 is given a dielectricconstant setting of 5.4, which setting corresponds to the setting givento the rate of flow controller. During operation, the refrigerationrequirements of the chiller are continuously adjusted so that the solidscontent of the slurry remains substantially constant at 40 mol percent.It the solids content of the slurry deviates from this value, thedielectric constant of the material also changes and there is anelectrical unbalance in the bridge circuit shown in FIGURE 4. The outputair pressure from the potentiometer controller also changes, therebyresetting the flow controller which in turn adjusts valve 40 in line 41.When the dielectric constant of the slurry decreases indicating anincrease in the solids content, the rate of flow controller is reset sothat it operates to cut back on the opening of valve 40, therebydecreasing the rate of evaporation of the propane. Conversely, if thedielectric constant of the slurry increases, indicating a decrease inthe solids content, the rate of flow controller is reset so that itoperates to increase the opening of valve 40 and thereby permit agreater rate of propane evaporation. When the dielectric constant of theslurry returns to its original value of 5.4, the potentiometercontroller operates to reset the rate of flow controller to its originalsetting.

The slurry containing 40 mol percent solids is passed from the chillerinto the crystal separation and purification column. The slurry is movedthrough the column by means of a piston into the filter section wheremother liquor containing 75 weight percent MVP is recovered at the rateof 9,982 pounds per day. The mass of crystals, as a result of the forceasserted thereon by the column piston, moves through the column towardthe melting Zone maintained in the end of the column at a temperature of20 R, which is above the melting point of the MVP crystals. A streamcontaining 95 mol percent MVP is withdrawn from the melting zone at therate of 9,983 pounds per day as the product of the process.

It will be apparent to those skilled in the art that variousmodifications of the invention can be made upon study of theaccompanying disclosure. Such modifications are believed to be clearlywithin the spirit and scope of the invention.

We claim:

1. In fractional crystallization apparatus comprising, in combination,an elongated purification column, means for melting crystals associatedwith one end of said column and conduit means for withdrawing producttherefrom, crystal forming means having coolant inlet means and coolantoutlet means connected thereto, feed inlet means connected to saidcrystal forming means, slurry withdrawal means connected to said crystalforming means, and to the end portion of said column opposite from thatwith which said crystal melting means is associated, and filtering meansin said column between said crystal melting means and the point ofconnection of said slurry withdrawal means to said column, theimprovement comprising means associated with said slurry Withdrawalmeans for measuring in situ the dielectric properties of slurry passedtherethrough and producing an output as a function of said dielectricproperties; and means responsive to said output for controlling the rateof flow of coolant through one of said coolant inlet and coolant outletmeans connected to said crystal forming means and thus maintain asubstantially constant crystal content in said slurry.

2. In fractional crystallization apparatus including a crystallizationchamber with feed inlet means and crystal slurry outlet means and meansfor passing cooling medium in heat exchange relation with the contentsof said chamber, the improvement comprising, in combination, adielectric probe within said chamber and positioned to be in contactwith crystal slurry formed in said chamber and adjacent said outletmeans, a motor valve associated with said means for passing coolingmedium to control the flow rate of said cooling medium and hence theamount of cooling in said chamber, and control means operativelyconnected to both said motor valve and said dielectric probe, saidcontrol means serving to operate said motor valve in response to signalsproduced through said dielectric probe.

3. In fractional crystallization apparatus including a crystallizationchamber having a feed inlet and a crystal slurry outlet, a channel forcarrying coolant in indirect heat exchange with fluid in said chamber,and a conduit connected to said channel and through which said coolantmust pass, the improvement comprising, in combination, a dielectricprobe within said chamber and positioned to be in contact with crystalslurry formed in said chamber and adjacent said outlet, electricalcircuit means including a power source for producing a signal as afunction of the dielectric value of slurry contacting said probe, amotor valve in said conduit, and control means operating said motorvalve in response to said signal.

4. Crystallization apparatus comprising, in combination, a chamberhaving inlet and outlet means through which can be passed materialhaving a crystallizable component, means for removing heat from materialWithin said chamber to form a crystal slurry, means for measuring thedielectric properties of said slurry in said outlet, and control meansconnected to said means for removing heat and to said means formeasuring dielectric properties making heat removal by the formerresponsive to variations in signal generated by the latter.

5. Crystallization apparatus comprising, in combination, acrystallization chamber, means for removing heat from material in saidchamber to form a crystal slurry, means for sensing the dielecticproperties of said slurry, and control means operatively connecting saidheat removing means and said dielectric sensing means and regulatingheat removal in response to variations in dielectric properties of saidslurry.

6. In combination with a crystal purification column connected to acrystallization means through a slurry conduit, said column comprisingoutlet means at the opposite end from said conduit, melting means atsaid opposite end adjacent said outlet, and filter means in said columnintermediate said conduit and said melting means, said crystallizationmeans having heat removal means associated therewith, the improvementwhich comprises, in combination a dielectric probe extending into saidslurry conduit,

means connected with said probe to detect variations in the dielectricconstant of slurry within said conduit and to generate a signalproportional to said dielectric constant,

and means to control said heat removal means in response to variationsin said signal and thus maintain said signal substantially constant.

References Cited by the Examiner UNITED STATES PATENTS 1,970,613 8/34-McDill 16539 2,472,409 6/49 Eisner 165-39 2,572,253 10/51 Fellows et a1.165--39 Pay 2l096 X Robinson et al. 2l096 X Englehardt 62139 Findlay23273 Tarr 260-674 X Miller 2l096 ROBERT A. OLEARY, Primary Examiner.

GEORGE D. MITCHELL, Examiner.

5. CRYSTALLIZATION APPARATUS COMPRISING, IN COMBINATION, ACRYSTALLIZATION CHAMBER, MEANS FOR REMOVING HEAT FROM MATERIAL IN SAIDCHAMBER TO FORM A CRYSTAL SLURRY, MEANS FOR SENSING THE DIELECTICPROPERTIES OF SAID SLURRY AND CONTROL MEANS OPERATIVELY CONNECTING SAIDHEAT REMOVING MEANS AND SAID DIELECTRIC SENSING MEANS AND REGULATINGHEAT REMOVAL IN RESPONSE TO VARIATIONS IN DIELECTRIC PROPERTIES OF SAIDSLURRY.